Composite structures have been known in the art for many years. Although composite structures can be formed in many different manners, one advantageous technique for forming composite structures is a fiber placement or automated collation process. According to conventional automated collation techniques, one or more ribbons of composite material, known as composite strands or tows, are laid down on a substrate. The substrate may be a tool or mandrel, but more conventionally, is formed of one or more underlying layers of composite material that have been previously laid down and compacted.
Conventional fiber placement processes in the formation of a part utilize a heat source to assist in the compaction of the plies of composite material at a localized nip point. In particular, the ribbons or tows of the composite material and the underlying substrate are heated at the nip point to increase resin tack while being subjected to compressive forces to ensure adhesion to the substrate. To complete the part, additional strips of composite material can be applied in a side-by-side manner to each layer and can be subjected to localized heat and pressure during the consolidation process.
Unfortunately, defects can occur during the placement of the composite strips onto the underlying composite structure. Such defects can include tow gaps, overlaps, dropped tows, puckers, and twists. Additionally, foreign objects and debris (FOD), such as resin balls and fuzz balls, can accumulate on a surface of the composite structure. Resin balls are small pieces of neat resin that build up on the surfaces of the fiber placement head as the preimpregnated tows pass through the guides and cutters. The resin balls become dislodged due to the motion and vibration of the fiber placement machine, and drop on to the surface of the ply. Subsequent courses of applied layers cover the resin ball and a resultant bump is created in the laminate whereat there may be no compaction of the tows. The fuzz balls are formed when fibers at the edges of the tows fray and break off as the tows are passed through the cutter assembly. The broken fibers collect in small clumps that fall onto the laminate and are covered by a subsequent layer.
Composite laminates fabricated by fiber placement processes are typically subjected to a 100 percent ply-by-ply visual inspection for both defects and FOD. Typically, these inspections are performed manually during which time the fiber placement machine is stopped and the process of laying materials halted until the inspection and subsequent repairs, if any, are completed. In the meantime, the fabrication process has been disadvantageously slowed by the manual inspection process and machine downtime associated therewith.
Current inspection systems are capable of identifying defects in a composite structure during the fabrication process without requiring machine stoppage for manual inspections. The inspection systems are capable of detecting and identifying FOD “in-process” or during the fabrication of a composite structure. This, in turn, eliminates the need for manual FOD inspections and the machine downtime associated therewith.
A split illumination technique has been introduced for the detection of flaws and FOD simultaneously. A first half of a viewing area, referred to as the bright field, is illuminated. A second half of the viewing area, referred to as the dark field, is not illuminated. The flaws in the composite structure are detectable within the bright field, but are indistinguishable in the dark field. The FOD is detectable in the dark field, but is indistinguishable in the bright field. Thus, the split illumination technique requires the use of dual illumination levels. The use of dual illumination levels complicates the inspection process by causing a single field to be viewed twice, which is time consuming. A single field must be viewed twice to inspect for both flaws and FOD.
Thus, there exists a need for an improved system and method of the detection and identification of flaws and FOD within a composite structure during the fabrication thereof that simplifies and minimizes the time involved in the inspection of that composite structure.